The invention concerns a superconducting magnet system for magnetic resonance spectrometers, wherein the magnet system comprises an actively shielded superconducting magnet for generating a homogeneous magnetic field H.sub.0 along a z-axis in a working volume, arranged about z=0, comprising a radially inner and a radially outer coil system, wherein the two coil systems carry approximately identical currents and have approximately identical opposite dipole moments, and wherein a shim system is provided comprising a superconducting shim coil system for correcting magnetic field inhomogeneities in the working volume, the z component of which is proportional to z.sup.2.
An actively shielded superconducting magnet system of this kind, comprising a Z.sup.2 shim is contained e.g. in the NMR Magnet System 500 SB UltraShield.TM. distributed by the company Bruker Magnetics and shown in the company leaflet dated May 15, 1999.
An actively shielded magnet system without Z.sup.2 shim is e.g. known from EP 0 144 171 A1.
To achieve a good resolution of the resonance lines in the spectrum by means of magnetic resonance spectroscopic methods, the magnetic field must have a good homogeneity in the sample volume. The basic homogeneity of the superconducting magnet can be optimized with the geometric arrangement of the field-generating magnet coils. With demanding applications, such as high-resolution nuclear magnetic resonance spectroscopy, the basic homogeneity of the magnet is, in most cases, insufficient, since e.g. deviations from the design occur due to production tolerances. To compensate residual inhomogeneities of the magnet, the magnet system comprises autonomous superconducting coils which can compensate field inhomogeneities with a particular geometric symmetry in the sample volume, so-called shims.
An example of such a shim is the Z.sup.2 shim which generates a field with a strength proportional to z.sup.2 along the magnet axis z. A problem with known Z.sup.2 shim means according to prior art is the fact that with large magnets, the shim must be wound within the magnet coils since it would be too weak at the outside. This reduces the space for magnet windings which makes the magnet more expensive and increases the stray field with the consequence that stray field shielding must be stronger for magnet systems with active stray field compensation.
Optimization of the Z.sup.2 shim can be achieved by reducing the space required for the Z.sup.2 shim coils which are wound radially within the magnet windings. This is possible without reducing the shim efficiency only by distributing the shim coils onto several radii. This gives rise to new problems, in particular in connection with the coupling of the shim with fluctuating external magnetic fields.
In contrast thereto, it is the underlying purpose of the present invention to optimize a magnet system of the above-described kind with as simple as possible means such that the above-mentioned problems do not occur.